Fix: Update deprecated DisqusShortname usage in Whiteplain Hugo theme

[robotAstray]

Description

I have been using your theme to create a site with the Hugo site generator and noticed a small bug that needs attention. I have opened this PR to resolve this issue.

Steps to Reproduce

1. Run the following command:

   hugo server --buildDraft

Error Message

```
ERROR deprecated: .Site.DisqusShortname was deprecated in Hugo v0.120.0 and will be removed in Hugo 0.134.0. Use .Site.Config.Services.Disqus.Shortname instead.
Built in 12 ms
Error: error building site: logged 1 error(s)
```

Software Version

• Hugo v0.134.0

Proposed Solution

This PR updates the theme by replacing .Site.DisqusShortname with .Site.Config.Services.Disqus.Shortname in the layouts/_default/single.html file.

[robotAstray]

@taikii I have reverted to color: #444444;on line 6

[taikii]

Thanks for the fix! I've merged this. 🎉